Harmonic complex tones (in which all frequency components are multiple of a common fundamental, F0) produce a strong pitch sensation that plays an important role in speech communication, music perception and the perceptual organization of acoustic scenes into auditory objects. Pitch perception and the ability to use differences in pitch to parse auditory scenes are often degraded in hearing impaired listeners and cochlear implant users. While the neural coding of pitch in the auditory nerve and cochlear nucleus is well understood, and pitch-selective neurons have been identified in the marmoset auditory cortex, hardly anything is known about the intervening processing stages that select and transform the multiple pitch cues available in the pattern of peripheral activity to ultimately create pitch selectivity in cortical neurons. The overall goal of the proposed research is to characterize the neural coding of pitch in the inferior colliculus (IC) (the principal nucleus in te auditory midbrain) through a combination of single-unit recording from awake rabbits and behavioral tests of F0 discrimination in the same animal model. Specific Aim 1 will test the hypothesis that the tonotopic pattern of activity in the IC provides more robust rate-place cues to resolved harmonics of complex tones compared to the auditory nerve. Specific Aim 2 will measure responses of IC neurons to complex tones in which either temporal envelope cues or spectral harmonicity cues are selectively suppressed in order to gain insight into the processing leading to pitch-sensitive responses. Specific Aim 3 will use a novel, immersive behavioral method to test the ability of rabbits to discriminate changes in F0 if harmonic complexes with missing fundamentals and to generalize the learned discrimination to new stimuli differing in either spectral composition or temporal envelope cues. Finally, Specific Aim 4 will test whether there is sufficient information in the responses of IC neurons to identify both F0s in a pair of concurrent complex tones, a situation that is frequently encountered in crowded rooms or when listening to symphonic music. Together, these aims will increase our basic understanding of neural mechanisms underlying pitch perception, which has been a topic of debate in auditory theory for nearly 200 years. They will also inform the rational design of new processing strategies for hearing aids and cochlear, brainstem and midbrain implants that would provide better music perception and improve speech perception in everyday acoustic environments with multiple sound sources.